The horny layer is the most external layer contacting with the exterior and protects the skin and body from physical and chemical hazard. The horny layer consists of 50% protein (called “keratin”), 20% fat, 23% water-soluble substance and 7% water. Because skin cells continue to be produced, old skin cells gradually move up to the horny layer, and the cells arriving at the horny layer are flattened dead cells, called “keratins”. Millions of dead keratinocytes are removed from the skin daily and replaced with new keratinocytes (skin turnover). The term “peeling” refers to peeling a layer of dead cells from the skin epidermis so as to renew the skin. The skin naturally forms the turnover cycle where old keratin is removed by itself and a new skin moves up. However, due to aging or environmental conditions, such as drying, UV light and stress, old keratin remains without being naturally removed. For this reason, peeling as an artificial method is used to remove old keratin, such that the skin renewal cycle is restored and fresh cells produced in a basal layer is moved upward, thus making the skin clear and clean.
Cosmetics for such peeling will now be described.
(1) Chemical Peeling Products
Chemical peeling products are products containing acid components and can show the most rapid effect. The principle of chemical peeling is applied to loosen the linkage between keratin in a thick horny layer on the skin by the action of acid, such that excessive keratin, which has not been regularly removed, is softened such that it is well removed by, for example, facial washing. Chemical peeling agents are also most frequently used in dermatological peeling.
When a chemical peeling product is used in a home, it is important to previously examine the kind and content of acid used and to select a product suitable for the skin. Also, because old keratin is artificially removed, the amount of keratin is temporarily reduced, such that the protection of the skin from the external environment can become weak. For this reason, when a person moves during the daytime after he uses a peeling product, it is preferable to use a UV screening agent together with the peeling product.
Active ingredients, which are mainly used in chemical peeling, are as follows.
(a) AHA (Alpha Hydroxy Acid)
AHA is the most typical chemical component for removing keratin, helps to remove the linkage between keratins in the horny layer by the action of acid, and shows the effect of softening keratin. Specifically, components corresponding to AHA are as follows.                Glycolic acid: This is representative of AHA and is most frequently used in cosmetics. It has excellent skin permeability and an excellent ability to remove keratin. Also, it is used mainly in dermatology.        Lactic acid: This is a natural moisturizing factor on the human skin surface and is frequently used next to glycolic acid in cosmetics. It is extracted from sour milk, tomato juice and the like and is used in dermatology.        
In addition, other examples of AHA include malic acid extracted from apples, citric acid from oranges, and tartaric acid from grape wine.
(b) BHA (Beta Hydroxy Acid)
BHA is a typical chemical component for removing keratin, like AHA, and a BHA component, which can be most frequently seen in cosmetics, is salicylic acid. Because BHA is oil soluble, unlike AHA, it effectively removes keratins excessively deposited at acne sites and easily penetrates into skin sites, which have increased sebum secretion and acne formation. Thus, it is frequently used in products for acne. Such chemical agents for removing keratin can show a good effect within a short period of time, but have disadvantages in that they tend to irritate the skin and their effect is not long-lasting.
(c) Enzymes
Because the horny layer contains much protein, the use of proteases can provide the effect of cutting the linkage between keratins present in the horny layer. This principle is applied to remove either old keratins coming from the exterior or non-uniform old keratins. Typically used enzymes include papain enzyme as vegetable protease. Keratin removal by enzyme shows low irritation, and thus is useful for controlling keratin in a sensitive skin, but has a disadvantage in that it is difficult to stabilize enzyme activity in cosmetics.
(2) Physical Peeling Products
Physical peeling products include various products, including scrub agents that remove keratin by causing friction with grains, and recent microdermabrasion products that employ microparticles along with additive active ingredients. The microdermabrasion products show the effect of removing old keratin by applying microparticles on the skin and then applying physical stimulation with hands. In dermatology, the skin is peeled off either by applying physical stimulation using an abrasion machine instead of using hands or by spraying microparticles directly on the skin using strong pressure. The physical peeling products are various depending on the shape and size of microparticles, but are products that apply physical stimulation, and thus it is necessary to select products containing particles suitable for skin sensitivity.
Keratinocytes are continuously formed, but old keratinocytes in the outermost layer are peeled off, and thus a layer of keratinocytes maintains a constant thickness. This phenomenon that the location of new cell layers changes is called “turnover”. Epidermal turnover in the normal skin takes about 4 weeks, although it changes depending on site or age. When this horny layer remains on the skin without being normally peeled off, the horny layer becomes thick and the facial color becomes dark. Also, impurities on the skin surface or in hair follicles are oxidized or degraded by oxygen or microorganisms and cause skin troubles such as inflammation. The main component of such impurities is keratin, which remains on the skin surface even after cells are dead, and proteins in sweat remain on the skin surface after the sweat comes out. Because keratin has high molecular weight and is insoluble and not easily degraded, it is not easily removed with a cleaning agent alone, but it can be effectively removed by degrading it into small fragments using enzyme. The use of stable protease can remove a keratin layer attached to the skin surface and make the skin softer.
Typical examples of proteases, which have recently been used in cosmetics until recently, include papain, bromelain and the like that are proteases of plant origin. However, it is known that such proteases do not effectively degrade keratin and lose their activity during extraction and transport processes or do not maintain their activity over a long period of time. Moreover, alpha hydroxy acid that is used for the purpose of effectively keratin on the skin surface shows a tendency to gradually decrease, because it causes skin irritation due to low pH and weakens the skin surface upon repeated use.